What is a Unary association in UML

Question

While I googled to understand the Unary associations, I got the following two explanations:

the first is:

A unary relationship is when both participants in the relationship are the same entity. For Example: Subjects may be prerequisites for other subjects, or one employee manages many Employees.

and the second is:

Class B knows about ClassA.

Class A does not know about ClassB.

Now lets look at the following example:

You can see the Person and Address relationship below. We call this relationship as has-a relationship since person has a address. So Person knows the address but address does not know anything about person

Am I misunderstanding something?

Answer 1

Merriam-webster defines Unary as

having, consisting of, or acting on a single element, item, or component

So the first explanation is correct one since this type of association acts on a single class. The term Unary however is not used in UML and might be confusing.
UML uses the term binary to indicate that an association has two ends, and ternary or n-ary to indicate an association has multiple ends. The Unary association you are talking about is actually a binary association to itself, also known as reflexive association.

Not to be confused with a Unidirectional association, which is an association that is only navigable (has an arrow) to one side.